Wells are extended into the earth's crust to desired subterranean locations, e.g., oil and/or gas bearing formations, through the applicaton of rotary drilling techniques. In the rotary drilling of a well, a drilling fluid is circulated through the well in order to remove cuttings from the bottom of the well. Typically, this is accomplished by pumping the drilling fluid downwardly through a drill string within the well and then upwardly to the surface of the well through the annulus surrounding the drill string. The drill cuttings are entrained in the drilling fluid and brought to the surface with the fluid and then screened out and discarded. In addition to removing cuttings, the drilling fluid also serves to cool and lubricate the drill bit and, in the case of systems employing downhole drill motors, it functions as a power fluid for the drill motor.
While in some cases gaseous drilling fluids may be employed, most rotary drilling techniques employ a liquid medium, termed a "drilling mud", which has various solids dissolved or suspended therein. The drilling mud may comprise either an aqueous-base mud or an oil-base mud. The liquid medium employed in an oil base drilling mud may be a relatively pure oil such as crude petroleum oil or diesel oil, or it may be an "invert" emulsion, a water-in-oil emulsion in which oil forms the continuous phase. Such oil-base drilling muds may be used to advantage where the formation being drilling is a suspected oil or gas producing zone or where shale formations which are unstable to water are encountered. In water-base drilling muds, the liquid medium employed typically is fresh water, salt water or an oil-in water-emulsion in which water forms the continuous phase.
Both water-base and oil-base drilling muds normally contain clays and/or other dispersed solids which are employed to impart desired rheological properties to the mud. The suspended solids employed in water-base drilling muds typically take the form of clays of the montmorillonite or illite groups. These clays are utilized to impart desirable thixotropic properties to the drilling mud and also serve to coat the walls of the well with a relatively impermeable sheath, commonly termed a filter cake, which retards the flow of fluid from the well into the surrounding subterranean formations. Exemplary of the clays which may be employed in water base drilling muds is bentonite, which is montmorillonite-type clay. The bentonite is dispersed within the aqueous base liquid as colloidal particles and imparts the desired degree of thixotropy. Clays such as those discribed above which have been rendered oleophilic by suitable treatment may be used in oil base drilling fluid. For example, the "bentones" which are formed by treating bentonites with long chain amines are widely used in oil base drilling muds to impart desirable rheological properties. Other materials which may be used for this purpose in oil-base drilling fluids include oxidized tall oil, lampblack and blown asphalt. In addition to the various colloidal materials such as mentioned above, a drilling fluid may also contain one or more weighting agents which function to increase the density of the fluid to a level which will offset high pressures encountered during the drilling operation. Examples of weighting agents which may be used in either water base or oil base drilling muds are heavy minerals such as barite and gelena.
One frequently encountered difficulty in drilling operations is the problem of lost returns in which part or all of the whole drilling fluid is not returned to the surface. The problem may range from only moderate losses of fluid to a total loss of drilling fluid such that none of it is returned to the surface. Where a formation or thief zone is identified in which unacceptably large amounts of drilling fluid is lost, such formation is commonly termed a "loss zone" or a "loss circulation zone". Loss circulation may occur when the well encounters a formation of unusually high permeability or one which has naturally occurring horizontal or verticle fractures or fissures. Also the formation may be fractured accidentally by the hydrostatic pressure exerted by the drilling mud, particularly when a change over to a relatively heavy mud is made in order to control high formation pressures.
Numerous techniques have been developed in order to control loss circulation. One common expedient where the loss circulation is not severe is to add various fluid loss agents which function to change the rheological properties of the drilling mud in order to increase its resistence to flow from the well bore into the formation. Such fluid loss agents include synthetic polymeric thickening agents such as partially hydrolyzed polyacrylamide or a polyelectrolite such as the ionic polysaccharide B-1459, available under the trade name "Kelzan". Other common naturally occurring fluid loss agents include various gums such as locust bean gum and guar gum, various starches, and carboxymethylcellulose (CMC) or carboxyethylcellulose (CEC).
Where the loss circulation is more severe, it is a normal practice to incorporate into the drilling mud various bulk materials which function as bridging agents. The use of bridging agents is described by Messenger, J. U. "Loss Circulation", Pen Well Publishing Company, Tulsa, Okla. 1981, particularly Chapter 4, and as there noted, they may be characterized as granular, fibrous, and laminated (flakes). Examples of granular materials are ground walnut shells and plastics, ground coal, gilsonite, neoprene, limestone, sulfur, ground fruit pits and expanded perlite. Fibrous materials include sawdust, prairie hay, bark, sugar cane fiber, shredded wood, leather, asbestos and cottonseed hulls. Laminar materials or flakes include wood shavings, cellophane and mica. As disclosed by Messenger at pages 28 and 29, a two-component blend of a granular bridging agent such as sawdust and a fibrous agent such as leather flock is more effective than either agent alone. Also, as further disclosed by Messenger at page 32, a suitable three component mixture comprises 1-6 parts granular agent, 2 parts of coarse-to-fine fiber agent, and 1 part of a flake agent. At page 38, the author discloses an oil-mud additive comprising three parts granular, two parts fiber and one part flake.
In addition to the use of particulate bridging agents, various other procedures may be employed in order to control lost circulation. Such procedures may be used in conjunction with bridging agents or separately thereof. For example, hydraulic cement slurries may be introduced into the loss circulation zone to form a hard plug. A diesel-oil-bentonite slurry may be circulated into or adjacent the loss circulation zone and hydrated to form a "soft plug" which plugs the fracture or other loss zone. Also, a diesel-oil-bentonite slurries containing a hydratable cement such as portland cement may be employed to form a plug which is initially highly gelled but which then sets to form a hard plug. As noted previously, such hard plugs or soft plugs may be employed alone or in conjunction with particulate bridging agents.
At the conclusion of the drilling procedure, a changeover often is made from the drilling mud to a completion fluid which is maintained in the hole during the final steps of drilling and completing the well. For example, when the productive formation is encountered, it is conventional practice to halt drilling, set and cement casing, and then continue to drill into the producing zone. During this final drilling procedure, a relatively clean oil base or water base liquid is maintained in the hole in order to offset the downhole pressures encountered. Such completion fluids may also be employed in the course of other operations conducted at the conclusion of the drilling procedures such as circulating in gravel suspensions to form a "gravel pack" at the bottom of the well.
After a well is completed and placed on production, it is often necessary to interrupt the normal operation of the well to carry out various well servicing or workover operations with a "workover fluid" present in the well. The workover fluid, like the well completion fluid, may be an oil-base or a water-base liquid medium which is employed to maintain the necessary pressure at the bottom of the well without damaging the formation.
Many of the problems encountered in the circulation of a drilling mud during the normal drilling of the well may also be experienced when a well servicing fluid service such as a completion fluid or a workover fluid is circulated into the well. Accordingly, it is often times desirable to incorporate loss circulation or filter loss agents in well circulation fluids other than drilling muds.